Post Combustion Carbon Dioxide Capture: Exploration of Prospective Solvents

Abstract

Global warming is one of the toughest challenge, humanity is currently facing. Temperature of the Earth’s surface is influenced through the process of absorption of heat radiation by greenhouse gases, like H2O, CO2, CH4 etc. In the prevailing state of technology, huge CO2 are thrown from thermal power plant installations, natural gas purification, and refinery off gases. Carbon dioxide gas being a greenhouse gas needs to be controlled in its emission from various point sources, which is under the purview of revamping current technology as a benign one. Different post-combustion CO2 capture techniques are available like, absorption (physical and chemical), adsorption (physical), membrane, and cryogenic separation. Among them, chemical absorption-desorption is the most mature and suitable technology available so far. In chemical absorption process, solvent plays most vital role, which is weighed in terms of its CO2 absorption capacity, absorption rate, energy penalty for regeneration, availability, degradation resistance, toxicity etc. Hence, selection of solvent as CO2 absorbent seeks paramount attention.
Various limitations including high regeneration energy, higher vapour pressure, limited CO2 loading capacity of common alkanolamines solvents like monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), 2-amino-2-methyl propanol (AMP) and their blends, necessitated the exploration of alternative CO2 absorbing solvents. Newer solvents having different spatial arrangements have been utilized to study the effect of their molecular structure in CO2 absorbing property. Among them, 2-diethylaminoethanol (DEAE) and 1-(2-hydroxyethyl)piperidine (HEP) and their blends with piperazine (PZ) performed as better CO2 absorbents in the screening of solvent experiments followed by detail solubility studies.
Amino acid salts are very promising because of their negligible vapour pressure, lesser toxicity, high absorption capacity and rate, thermal stability etc. Pottasium salt of arginine (KArg) and lysine (KLys) revealed the highest apparent rate and capacity and potassium salt of 2-aminoisobutyric acid (KAmib) possesses moderate apparent rate and high capacity in the screening experiments among amino acid salts. KAmib, being a sterically hindered primary amino acid (similar structure like 2-amino-2-methyl-1-propanol (AMP)) salt; forms unstable carbamate thus possesses high capacity of CO2 absorption and lower regeneration energy. Due to high apparent rate constant of KArg, KLys and PZ compounds, they were used as rate promoters to enhance the CO2 absorption rate of KAmib solvent successfully.
Absorption of CO2 in amine based solvent is a regenerative and chemical absorption process. Regeneration energy varies depending upon the solvent formulation and is directly related with enthalpy of CO2 absorption of that solvent. Enthalpy of CO2 absorption is measured through calorimetric experiments. Interestingly, enthalpy of CO2 absorption could be precisely predicted from thermodynamic model using equilibrium CO2 solubility data. Since no experimental or predicted enthalpy of CO2 absorption in DEAE, HEP and their blends with PZ are available in the open literature, prediction of enthalpy from activity coefficient based model on solubility seems to be a significant contribution of this dissertation.
Limitation of regenerative absorption process could be overcome by using some water soluble organic solvents (acetone and tetrahydrofuran (THF)) having very low dielectric constant compared to water at low temperature for solvent regeneration. Solvent regeneration by this technique is only possible when the CO2 rich solution contains only bicarbonate and carbonate ions; not carbamate ions. CO2 rich DEAE solution was used for the study of CO2 regeneration using organic solvents like acetone/THF. This is one of the directional aspects of the present dissertation, which deserves further attention.
Besides experimental study to explore a suitable solvent for gas treating process, thermodynamic modelling plays important role to complement experimental results. Deshmukh and Mather model, one of the efficient activity coefficient based models describing multi-component and multi-phase fluid phase equilibria was used to correlate the vapour-liquid equilibrium data generated in the work. Liquid phase speciation for both CO2 loaded single and blended amine solutions, and enthalpy of CO2 absorption were predicted effectively using the developed model in MATLAB platform.
For developing Thermodynamic equilibrium and kinetic rate models, physicochemical parameters of those amine based solvents are required. In view of this, physicochemical properties like density and viscosity of single and blended alkanolamines over wide range of temperature, relative amine compositions were generated and correlated using thermodynamic framework, which are useful contribution to the design database of sour gas treating process.